Within a gas turbine engine it will be appreciated that a compressor stage presents an airflow to a combustion chamber where high temperatures and gas flows allow a turbine system to drive the engine. In order to achieve stable and efficient combustion it is desirable to ensure that there is a suitable air flow within the combustion stage. In such circumstances, a diffuser arrangement is provided in order to reduce airflow velocity and increase static pressure. In general terms, air is diffused or bled from the output compressor air flow. This diffused air is utilised for cooling and other purposes about the engine.
It is important when diffusing or bleeding air from the compressor stage air flow that such diffusion is achieved most efficiently and with the least degree of additional constructional complexity. It is necessary that the flow within the diffuser remains attached in order to achieve efficient diffusion—i.e. sufficient reduction in dynamic pressure and redistribution of the flow in order to achieve efficient combustion. Furthermore, in providing for such bleeding or diffusion it is necessary that the air flow from the compressor stage is maintained for efficient combustion. Initially, divergence ducts were provided in order to achieve diffusion but these may not be able to achieve desired and required higher rates of diffusion whilst the flow remains attached. It should also be understood that it is desirable to achieve a shorter engine length and such diffusion arrangements may be difficult to incorporate within the desired engine length.
An example of a previous diffuser arrangement is illustrated in European Patent Application No. 00306279.1 (Rolls-Royce Plc). In this previous diffuser arrangement air is taken from the air flow driven by the compressor using a relatively complex diffuser arrangement which must be constructed or fabricated before the combustion chamber. Clearly, incorporation of relatively complex structural features add to costs and engineering complexity.